Method of Making Semiconductor X-ray Detectors

ABSTRACT

Disclosed herein is a method of making an apparatus suitable for detecting x-ray, the method comprising: obtaining a substrate having a first surface and a second surface, wherein the substrate comprises an electronics system in or on the substrate, wherein the substrate comprises a plurality of electric contacts are on the first surface; obtaining a first chip comprising a first X-ray absorption layer, wherein the first X-ray absorption layer comprises an electrode; bonding the first chip to the substrate such that the electrode of the first X-ray absorption layer is electrically connected to at least one of the electrical contacts.

TECHNICAL FIELD

The disclosure herein relates to X-ray detectors, particularly relatesto methods of making semiconductor X-ray detectors.

BACKGROUND

X-ray detectors may be devices used to measure the flux, spatialdistribution, spectrum or other properties of X-rays.

X-ray detectors may be used for many applications. One importantapplication is imaging. X-ray imaging is a radiography technique and canbe used to reveal the internal structure of a non-uniformly composed andopaque object such as the human body.

Early X-ray detectors for imaging include photographic plates andphotographic films. A photographic plate may be a glass plate with acoating of light-sensitive emulsion. Although photographic plates werereplaced by photographic films, they may still be used in specialsituations due to the superior quality they offer and their extremestability. A photographic film may be a plastic film (e.g., a strip orsheet) with a coating of light-sensitive emulsion.

In the 1980s, photostimulable phosphor plates (PSP plates) becameavailable. A PSP plate may contain a phosphor material with colorcenters in its lattice. When the PSP plate is exposed to X-ray,electrons excited by X-ray are trapped in the color centers until theyare stimulated by a laser beam scanning over the plate surface. As theplate is scanned by laser, trapped excited electrons give off light,which is collected by a photomultiplier tube. The collected light isconverted into a digital image. In contrast to photographic plates andphotographic films, PSP plates can be reused.

Another kind of X-ray detectors are X-ray image intensifiers. Componentsof an X-ray image intensifier are usually sealed in a vacuum. Incontrast to photographic plates, photographic films, and PSP plates,X-ray image intensifiers may produce real-time images, i.e., do notrequire post-exposure processing to produce images. X-ray first hits aninput phosphor (e.g., cesium iodide) and is converted to visible light.The visible light then hits a photocathode (e.g., a thin metal layercontaining cesium and antimony compounds) and causes emission ofelectrons. The number of emitted electrons is proportional to theintensity of the incident X-ray. The emitted electrons are projected,through electron optics, onto an output phosphor and cause the outputphosphor to produce a visible-light image.

Scintillators operate somewhat similarly to X-ray image intensifiers inthat scintillators (e.g., sodium iodide) absorb X-ray and emit visiblelight, which can then be detected by a suitable image sensor for visiblelight. In scintillators, the visible light spreads and scatters in alldirections and thus reduces spatial resolution. Reducing thescintillator thickness helps to improve the spatial resolution but alsoreduces absorption of X-ray. A scintillator thus has to strike acompromise between absorption efficiency and resolution.

Semiconductor X-ray detectors largely overcome this problem by directconversion of X-ray into electric signals. A semiconductor X-raydetector may include a semiconductor layer that absorbs X-ray inwavelengths of interest. When an X-ray photon is absorbed in thesemiconductor layer, multiple charge carriers (e.g., electrons andholes) are generated and swept under an electric field towardselectrical contacts on the semiconductor layer. Cumbersome heatmanagement required in currently available semiconductor X-ray detectors(e.g., Medipix) can make a detector with a large area and a large numberof pixels difficult or impossible to produce.

SUMMARY

Disclosed herein is a method of making an apparatus suitable fordetecting x-ray, the method comprising: obtaining a substrate having afirst surface and a second surface, wherein the substrate comprises anelectronics system in or on the substrate, wherein the substratecomprises a plurality of electric contacts are on the first surface;obtaining a first chip comprising a first X-ray absorption layer,wherein the first X-ray absorption layer comprises an electrode; bondingthe first chip to the substrate such that the electrode of the firstX-ray absorption layer is electrically connected to at least one of theelectrical contacts.

According to an embodiment, the method further comprises mounting abacking substrate to the first chip such that the first chip issandwiched between the backing substrate and the substrate.

According to an embodiment, the method further comprises obtaining asecond chip comprising a second X-ray absorption layer, wherein thesecond X-ray absorption layer comprises an electrode, and bonding thesecond chip to the substrate such that the electrode of the second X-rayabsorption layer is electrically connected to at least one of theelectrical contacts.

According to an embodiment, a gap between the first chip and the secondchip is less than 100 microns.

According to an embodiment, the first chip is smaller in area than thesubstrate.

According to an embodiment, a ratio between a thermal expansioncoefficient of the first chip and a thermal expansion coefficient of thesubstrate is two or more.

According to an embodiment, the X-ray absorption layer comprisessilicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof.

According to an embodiment, the X-ray absorption layer is doped withchromium.

According to an embodiment, the X-ray absorption layer has a thicknessof 200 microns or less.

According to an embodiment, the first chip comprises a redistributionlayer (RDL) on the second surface.

According to an embodiment, the first chip comprises a via, wherein thevia extends from the first surface to the second surface.

According to an embodiment, the electronics system comprises: a firstvoltage comparator configured to compare a voltage of the electrode to afirst threshold; a second voltage comparator configured to compare thevoltage to a second threshold; a counter configured to register a numberof X-ray photons reaching the X-ray absorption layer; a controller;wherein the controller is configured to start a time delay from a timeat which the first voltage comparator determines that an absolute valueof the voltage equals or exceeds an absolute value of the firstthreshold; wherein the controller is configured to activate the secondvoltage comparator during the time delay; wherein the controller isconfigured to cause the number registered by the counter to increase byone, if the second voltage comparator determines that an absolute valueof the voltage equals or exceeds an absolute value of the secondthreshold.

According to an embodiment, the electronics system further comprises acapacitor module electrically connected to the electrode of the firstX-ray absorption layer, wherein the capacitor module is configured tocollect charge carriers from the electrode of the first X-ray absorptionlayer.

According to an embodiment, the controller is configured to activate thesecond voltage comparator at a beginning or expiration of the timedelay.

According to an embodiment, the electronics system further comprises avoltmeter, wherein the controller is configured to cause the voltmeterto measure the voltage upon expiration of the time delay.

According to an embodiment, the controller is configured to determine anX-ray photon energy based on a value of the voltage measured uponexpiration of the time delay.

According to an embodiment, the controller is configured to connect theelectrode of the first X-ray absorption layer to an electrical ground.

According to an embodiment, a rate of change of the voltage issubstantially zero at expiration of the time delay.

According to an embodiment, a rate of change of the voltage issubstantially non-zero at expiration of the time delay.

According to an embodiment, the X-ray absorption layer comprises adiode.

BRIEF DESCRIPTION OF FIGURES

FIG. 1A schematically shows a semiconductor X-ray detector, according toan embodiment.

FIG. 1B shows a semiconductor X-ray detector 100, according anembodiment.

FIG. 2 shows an exemplary top view of a portion of the detector in FIG.1A, according to an embodiment.

FIG. 3 schematically shows the electronics layer 120 according to anembodiment.

FIG. 4A schematically shows direct bonding between an X-ray absorptionlayer and an electronic layer.

FIG. 4B schematically shows flip chip bonding between an X-rayabsorption layer and an electronic layer.

FIG. 4C schematically shows the electronic layer according to anembodiment.

FIG. 4D schematically shows that multiple chips may be obtained and eachof the chips includes an X-ray absorption layer such as the X-rayabsorption layer shown in FIG. 1A, FIG. 1B, FIG. 2, FIG. 3, FIG. 4A orFIG. 4B.

FIG. 4E shows that the chips may be bonded to the substrate of theelectronic layer.

FIG. 4F and FIG. 4G schematically show that a backing substrate may bemounted to the chips such that the chips are sandwiched between thebacking substrate and the substrate of the electronic layer.

FIG. 5 schematically shows a bottom view of the electronic layer.

FIG. 6A shows that the electronics layer as shown in FIG. 3 allowsstacking multiple semiconductor X-ray detectors.

FIG. 6B schematically shows a top view of multiple semiconductor X-raydetectors 100 stacked.

FIG. 7A and FIG. 7B each show a component diagram of an electronicsystem of the detector in FIG. 1A or FIG. 1B, according to anembodiment.

FIG. 8 schematically shows a temporal change of the electric currentflowing through an electrode (upper curve) of a diode or an electricalcontact of a resistor of an X-ray absorption layer exposed to X-ray, theelectric current caused by charge carriers generated by an X-ray photonincident on the X-ray absorption layer, and a corresponding temporalchange of the voltage of the electrode (lower curve), according to anembodiment.

FIG. 9 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by noise (e.g., darkcurrent), and a corresponding temporal change of the voltage of theelectrode (lower curve), in the electronic system operating in the wayshown in FIG. 8, according to an embodiment.

FIG. 10 schematically shows a temporal change of the electric currentflowing through an electrode (upper curve) of the X-ray absorption layerexposed to X-ray, the electric current caused by charge carriersgenerated by an X-ray photon incident on the X-ray absorption layer, anda corresponding temporal change of the voltage of the electrode (lowercurve), when the electronic system operates to detect incident X-rayphotons at a higher rate, according to an embodiment.

FIG. 11 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by noise (e.g., darkcurrent), and a corresponding temporal change of the voltage of theelectrode (lower curve), in the electronic system operating in the wayshown in FIG. 10, according to an embodiment.

FIG. 12 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by charge carriersgenerated by a series of X-ray photons incident on the X-ray absorptionlayer, and a corresponding temporal change of the voltage of theelectrode, in the electronic system operating in the way shown in FIG.10 with RST expires before t_(e), according to an embodiment.

FIG. 13 schematically shows a system comprising the semiconductor X-raydetector described herein, suitable for medical imaging such as chestX-ray radiography, abdominal X-ray radiography, etc., according to anembodiment

FIG. 14 schematically shows a system comprising the semiconductor X-raydetector described herein suitable for dental X-ray radiography,according to an embodiment.

FIG. 15 schematically shows a cargo scanning or non-intrusive inspection(NII) system comprising the semiconductor X-ray detector describedherein, according to an embodiment.

FIG. 16 schematically shows another cargo scanning or non-intrusiveinspection (NII) system comprising the semiconductor X-ray detectordescribed herein, according to an embodiment.

FIG. 17 schematically shows a full-body scanner system comprising thesemiconductor X-ray detector described herein, according to anembodiment.

FIG. 18 schematically shows an X-ray computed tomography (X-ray CT)system comprising the semiconductor X-ray detector described herein,according to an embodiment.

FIG. 19 schematically shows an electron microscope comprising thesemiconductor X-ray detector described herein, according to anembodiment.

DETAILED DESCRIPTION

FIG. 1A schematically shows a semiconductor X-ray detector 100,according an embodiment. The semiconductor X-ray detector 100 mayinclude an X-ray absorption layer 110 and an electronics layer 120(e.g., an ASIC) for processing or analyzing electrical signals incidentX-ray generates in the X-ray absorption layer 110. In an embodiment, thesemiconductor X-ray detector 100 does not comprise a scintillator. TheX-ray absorption layer 110 may include a semiconductor material such as,silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof. Thesemiconductor may have a high mass attenuation coefficient for the X-rayenergy of interest. The X-ray absorption layer 110 may include one ormore diodes (e.g., p-i-n or p-n) formed by a first doped region 111, oneor more discrete regions 114 of a second doped region 113. The seconddoped region 113 may be separated from the first doped region 111 by anoptional the intrinsic region 112. The discrete portions 114 areseparated from one another by the first doped region 111 or theintrinsic region 112. The first doped region 111 and the second dopedregion 113 have opposite types of doping (e.g., region 111 is p-type andregion 113 is n-type, or region 111 is n-type and region 113 is p-type).In the example in FIG. 1A, each of the discrete regions 114 of thesecond doped region 113 forms a diode with the first doped region 111and the optional intrinsic region 112. Namely, in the example in FIG.1A, the X-ray absorption layer 110 has a plurality of diodes having thefirst doped region 111 as a shared electrode. The first doped region 111may also have discrete portions.

FIG. 1B shows a semiconductor X-ray detector 100, according anembodiment. The semiconductor X-ray detector 100 may include an X-rayabsorption layer 110 and an electronics layer 120 (e.g., an ASIC) forprocessing or analyzing electrical signals incident X-ray generates inthe X-ray absorption layer 110. In an embodiment, the semiconductorX-ray detector 100 does not comprise a scintillator. The X-rayabsorption layer 110 may include a semiconductor material such as,silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof. Thesemiconductor may have a high mass attenuation coefficient for the X-rayenergy of interest. The X-ray absorption layer 110 may not include adiode but includes a resistor.

When an X-ray photon hits the X-ray absorption layer 110 includingdiodes, it may be absorbed and generate one or more charge carriers by anumber of mechanisms. An X-ray photon may generate 10 to 100000 chargecarriers. The charge carriers may drift to the electrodes of one of thediodes under an electric field. The field may be an external electricfield. The electrical contact 119B may include discrete portions each ofwhich is in electrical contact with the discrete regions 114. In anembodiment, the charge carriers may drift in directions such that thecharge carriers generated by a single X-ray photon are not substantiallyshared by two different discrete regions 114 (“not substantially shared”here means less than 5%, less than 2% or less than 1% of these chargecarriers flow to a different one of the discrete regions 114 than therest of the charge carriers). In an embodiment, the charge carriersgenerated by a single X-ray photon can be shared by two differentdiscrete regions 114. FIG. 2 shows an exemplary top view of a portion ofthe device 100 with a 4-by-4 array of discrete regions 114. Chargecarriers generated by an X-ray photon incident around the footprint ofone of these discrete regions 114 are not substantially shared withanother of these discrete regions 114. The area around a discrete region114 in which substantially all (more than 95%, more than 98% or morethan 99% of) charge carriers generated by an X-ray photon incidenttherein flow to the discrete region 114 is called a pixel associatedwith the discrete region 114. Namely, less than 5%, less than 2% or lessthan 1% of these charge carriers flow beyond the pixel. By measuring thedrift current flowing into each of the discrete regions 114, or the rateof change of the voltage of each of the discrete regions 114, the numberof X-ray photons absorbed (which relates to the incident X-rayintensity) and/or the energies thereof in the pixels associated with thediscrete regions 114 may be determined. Thus, the spatial distribution(e.g., an image) of incident X-ray intensity may be determined byindividually measuring the drift current into each one of an array ofdiscrete regions 114 or measuring the rate of change of the voltage ofeach one of an array of discrete regions 114. The pixels may beorganized in any suitable array, such as, a square array, a triangulararray and a honeycomb array. The pixels may have any suitable shape,such as, circular, triangular, square, rectangular, and hexangular. Thepixels may be individually addressable.

When an X-ray photon hits the X-ray absorption layer 110 including aresistor but not diodes, it may be absorbed and generate one or morecharge carriers by a number of mechanisms. An X-ray photon may generate10 to 100000 charge carriers. The charge carriers may drift to theelectrical contacts 119A and 119B under an electric field. The field maybe an external electric field. The electrical contact 119B includesdiscrete portions. In an embodiment, the charge carriers may drift indirections such that the charge carriers generated by a single X-rayphoton are not substantially shared by two different discrete portionsof the electrical contact 119B (“not substantially shared” here meansless than 5%, less than 2% or less than 1% of these charge carriers flowto a different one of the discrete portions than the rest of the chargecarriers). In an embodiment, the charge carriers generated by a singleX-ray photon can be shared by two different discrete portions of theelectrical contact 119B. Charge carriers generated by an X-ray photonincident around the footprint of one of these discrete portions of theelectrical contact 119B are not substantially shared with another ofthese discrete portions of the electrical contact 119B. The area arounda discrete portion of the electrical contact 119B in which substantiallyall (more than 95%, more than 98% or more than 99% of) charge carriersgenerated by an X-ray photon incident therein flow to the discreteportion of the electrical contact 119B is called a pixel associated withthe discrete portion of the electrical contact 119B. Namely, less than5%, less than 2% or less than 1% of these charge carriers flow beyondthe pixel associated with the one discrete portion of the electricalcontact 119B. By measuring the drift current flowing into each of thediscrete portion of the electrical contact 119B, or the rate of changeof the voltage of each of the discrete portions of the electricalcontact 119B, the number of X-ray photons absorbed (which relates to theincident X-ray intensity) and/or the energies thereof in the pixelsassociated with the discrete portions of the electrical contact 119B maybe determined. Thus, the spatial distribution (e.g., an image) ofincident X-ray intensity may be determined by individually measuring thedrift current into each one of an array of discrete portions of theelectrical contact 119B or measuring the rate of change of the voltageof each one of an array of discrete portions of the electrical contact119B. The pixels may be organized in any suitable array, such as, asquare array, a triangular array and a honeycomb array. The pixels mayhave any suitable shape, such as, circular, triangular, square,rectangular, and hexangular. The pixels may be individually addressable.

The electronics layer 120 may include an electronic system 121 suitablefor processing or interpreting signals generated by X-ray photonsincident on the X-ray absorption layer 110. The electronic system 121may include an analog circuitry such as a filter network, amplifiers,integrators, and comparators, or a digital circuitry such as amicroprocessors, and memory. The electronic system 121 may includecomponents shared by the pixels or components dedicated to a singlepixel. For example, the electronic system 121 may include an amplifierdedicated to each pixel and a microprocessor shared among all thepixels. The electronic system 121 may be electrically connected to thepixels by vias 131. Space among the vias may be filled with a fillermaterial 130, which may increase the mechanical stability of theconnection of the electronics layer 120 to the X-ray absorption layer110. Other bonding techniques are possible to connect the electronicsystem 121 to the pixels without using vias.

FIG. 3 schematically shows the electronics layer 120 according to anembodiment. The electronic layer 120 comprises a substrate 122 having afirst surface 124 and a second surface 128. A “surface” as used hereinis not necessarily exposed, but can be buried wholly or partially. Theelectronic layer 120 comprises one or more electric contacts 125 on thefirst surface 124. The one or more electric contacts 125 may beconfigured to be electrically connected to one or more electrodes of theX-ray absorption layer 110. The electronics system 121 may be in or onthe substrate 122. The electronic layer 120 comprises one or more vias126 extending from the first surface 124 to the second surface 128. Theelectronic layer 120 comprises a redistribution layer (RDL) 123 on thesecond surface 128. The RDL 123 may comprise one or more transmissionlines 127. The electronics system 121 is electrically connected to theelectric contacts 125 and the transmission lines 127 through the vias126.

The substrate 122 may be a thinned substrate. For example, the substratemay have at thickness of 750 microns or less, 200 microns or less, 100microns or less, 50 microns or less, 20 microns or less, or 5 microns orless. The substrate 122 may be a silicon substrate or a substrate orother suitable semiconductor or insulator. The substrate 122 may beproduced by grinding a thicker substrate to a desired thickness.

The one or more electric contacts 125 may be a layer of metal or dopedsemiconductor. For example, the electric contacts 125 may be gold,copper, platinum, palladium, doped silicon, etc.

The vias 126 pass through the substrate 122 and electrically connectelectrical components (e.g., the electrical contacts 125) on the firstsurface 124 to electrical components (e.g., the RDL) on the secondsurface 128. The vias 126 are sometimes referred to as “through-siliconvias” although they may be fabricated in substrates of materials otherthan silicon.

The RDL 123 may comprise one or more transmission lines 127. Thetransmission lines 127 electrically connect electrical components (e.g.,the vias 126) in the substrate 122 to bonding pads at other locations onthe substrate 122. The transmission lines 127 may be electricallyisolated from the substrate 122 except at certain vias 126 and certainbonding pads. The transmission lines 127 may be a material (e.g., Al)with small mass attenuation coefficient for the X-ray energy ofinterest. The RDL 123 may redistribute electrical connections to moreconvenient locations.

FIG. 4A schematically shows direct bonding between the X-ray absorptionlayer 110 and the electronic layer 120 at electrodes such as thediscrete regions 114 and the electrical contacts 125. Direct bonding isa wafer bonding process without any additional intermediate layers(e.g., solder bumps). The bonding process is based on chemical bondsbetween two surfaces. Direct bonding may be at elevated temperature butnot necessarily so.

FIG. 4B schematically shows flip chip bonding between the X-rayabsorption layer 110 and the electronic layer 120 at electrodes such asthe discrete regions 114 and the electrical contacts 125. Flip chipbonding uses solder bumps 199 deposited onto contact pads (e.g., theelectrodes of the X-ray absorption layer 110 or the electrical contacts125). Either the X-ray absorption layer 110 or the electronic layer 120is flipped over and the electrodes of the X-ray absorption layer 110 arealigned to the electrical contacts 125. The solder bumps 199 may bemelted to solder the electrodes and the electrical contacts 125together. Any void space among the solder bumps 199 may be filled withan insulating material.

FIG. 4C schematically shows the electronic layer 120 according to anembodiment. The substrate 122 of the electronic layer 120 has multipleelectric contacts 125 on the first surface 124. The multiple electriccontacts 125 may be organized into multiple regions 129. The electronicssystem 121 may be in or on the substrate 122.

FIG. 4D schematically shows that multiple chips 189 may be obtained andeach of the chips 189 includes an X-ray absorption layer such as theX-ray absorption layer 110 shown in FIG. 1, FIG. 2, FIG. 3, FIG. 4A orFIG. 4B. The X-ray absorption layer in each of the chips 189 has anelectrode.

FIG. 4E shows that the chips 189 may be bonded to the substrate 122using a suitable bonding method such as flip chip bonding or directingbonding as shown in FIG. 4A and FIG. 4B. In an embodiment, each of thechips 189 is bonded to one of the areas 129. The electrode of each ofthe chips 189 is electrically connected to at least one of theelectrical contacts 125. The gap between two neighboring chips 189,after the chips 189 are bonded to the substrate 122, may be 100 micronsor less. The chips 189 may be smaller in area than the substrate 122.The chips 189 may be arranged as an array after being bonded to thesubstrate 122. The smaller sizes of the chips 189 relative to thesubstrate 122 may help accommodating the difference in thermal expansioncoefficients of the chips 189 and the substrate 122. A ratio between thethermal expansion coefficient of the chips 189 and the thermal expansioncoefficient of the substrate 122 may be two or more. The X-rayabsorption layer in the chips 189 may be 200 microns thick or less, 100microns thick or less or 50 microns thick or less. Smaller thickness ofthe X-ray absorption layer reduces the chance that the charge carriersare trapped by defects in the X-ray absorption layer and thus increasesthe charge collection efficiency (CCE) by the electronic system 121. TheX-ray absorption layer in the chips 189 may be a material doped withchromium, especially when the material is GaAs. Chromium doping in GaAsmay reduce the concentration of EL2 defects in GaAs and thus allowshigher thickness of the X-ray absorption layer (thus higher absorptionefficiency) without losing too many charge carriers to defects. Thesubstrate 122 may have vias such as vias 126 shown in FIG. 3, and a RDLsuch as the RDL shown in FIG. 3 or FIG. 5.

FIG. 4F and FIG. 4G schematically show that a backing substrate 150 maybe mounted to the chips 189 such that the chips 189 are sandwichedbetween the backing substrate 150 and the substrate 122. The backingsubstrate 150 may provide mechanical support to the chips 189 and thesubstrate 122. The backing substrate 150 may be a material (e.g.,silicon, silicon oxide) with small mass attenuation coefficient for theX-ray energy of interest. The backing substrate 150 may also serve as anelectrode to apply an electric field across the thickness of the X-rayabsorption layer. There might be a thin layer of conductor between thebacking substrate 150 and the X-ray absorption layer for betterconduction.

FIG. 5 schematically shows a bottom view of the RDL 123, with othercomponents obstructing the view omitted. The transmission lines 127 canbe seen to electrically connect to vias 126 and redistribute vias 126 toother locations.

FIG. 6A shows that the electronics layer 120 as shown in FIG. 3 allowsstacking multiple semiconductor X-ray detectors 100 because the RDL 123and the vias 126 facilitate routing of signal paths through multiplelayers and because the electronic system 121 as described below may havelow enough power consumption to eliminate bulky cooling mechanisms. Themultiple semiconductor X-ray detectors 100 in the stack do not have beidentical. For example, the multiple semiconductor X-ray detectors 100may differ in thickness, structure, or material.

FIG. 6B schematically shows a top view of multiple semiconductor X-raydetectors 100 stacked. Each layer may have multiple detectors 100 tiledto cover a larger area. The tiled detectors 100 in one layer can bestaggered relative to the tiled detectors 100 in another layer, whichmay eliminate gaps in which incident X-ray photons cannot be detected.

According to an embodiment, multiple semiconductor X-ray detectors 100may be tiled side-by-side to form a larger detector. Each of themultiple detectors may have a single or multiple chips. For example, forthe application in mammography, the absorption layer may be made on asingle silicon wafer, which may be bonded to an electronic layer made onanother single silicon wafer. Four to six such detectors may be tiledside-by-side like tiles to form a tiled detector large enough to takeX-ray images of a human breast. Multiple tiled detectors may be stackedwith the gaps within each layer staggered.

According to an embodiment, the semiconductor X-ray detector 100 may befabricated using a method including: obtaining an X-ray absorption layercomprising an electrode; obtaining an electronics layer, the electronicslayer comprising: a substrate having a first surface and a secondsurface, an electronics system in or on the substrate, an electriccontact on the first surface, a via, and a redistribution layer (RDL) onthe second surface; bonding the X-ray absorption layer and theelectronics layer such that the electrode is electrically connected tothe electric contact; wherein the RDL comprises a transmission line;wherein the via extends from the first surface to the second surface;wherein the electronics system is electrically connected to the electriccontact and the transmission line through the via.

FIG. 7A and FIG. 7B each show a component diagram of the electronicsystem 121, according to an embodiment. The electronic system 121 mayinclude a first voltage comparator 301, a second voltage comparator 302,a counter 320, a switch 305, a voltmeter 306 and a controller 310.

The first voltage comparator 301 is configured to compare the voltage ofan electrode of a diode 300 to a first threshold. The diode may be adiode formed by the first doped region 111, one of the discrete regions114 of the second doped region 113, and the optional intrinsic region112. Alternatively, the first voltage comparator 301 is configured tocompare the voltage of an electrical contact (e.g., a discrete portionof electrical contact 119B) to a first threshold. The first voltagecomparator 301 may be configured to monitor the voltage directly, orcalculate the voltage by integrating an electric current flowing throughthe diode or electrical contact over a period of time. The first voltagecomparator 301 may be controllably activated or deactivated by thecontroller 310. The first voltage comparator 301 may be a continuouscomparator. Namely, the first voltage comparator 301 may be configuredto be activated continuously, and monitor the voltage continuously. Thefirst voltage comparator 301 configured as a continuous comparatorreduces the chance that the system 121 misses signals generated by anincident X-ray photon. The first voltage comparator 301 configured as acontinuous comparator is especially suitable when the incident X-rayintensity is relatively high. The first voltage comparator 301 may be aclocked comparator, which has the benefit of lower power consumption.The first voltage comparator 301 configured as a clocked comparator maycause the system 121 to miss signals generated by some incident X-rayphotons. When the incident X-ray intensity is low, the chance of missingan incident X-ray photon is low because the time interval between twosuccessive photons is relatively long. Therefore, the first voltagecomparator 301 configured as a clocked comparator is especially suitablewhen the incident X-ray intensity is relatively low. The first thresholdmay be 5-10%, 10%-20%, 20-30%, 30-40% or 40-50% of the maximum voltageone incident X-ray photon may generate in the diode or the resistor. Themaximum voltage may depend on the energy of the incident X-ray photon(i.e., the wavelength of the incident X-ray), the material of the X-rayabsorption layer 110, and other factors. For example, the firstthreshold may be 50 mV, 100 mV, 150 mV, or 200 mV.

The second voltage comparator 302 is configured to compare the voltageto a second threshold. The second voltage comparator 302 may beconfigured to monitor the voltage directly, or calculate the voltage byintegrating an electric current flowing through the diode or theelectrical contact over a period of time. The second voltage comparator302 may be a continuous comparator. The second voltage comparator 302may be controllably activate or deactivated by the controller 310. Whenthe second voltage comparator 302 is deactivated, the power consumptionof the second voltage comparator 302 may be less than 1%, less than 5%,less than 10% or less than 20% of the power consumption when the secondvoltage comparator 302 is activated. The absolute value of the secondthreshold is greater than the absolute value of the first threshold. Asused herein, the term “absolute value” or “modulus” |x| of a real numberx is the non-negative value of x without regard to its sign. Namely,

${x} = \{ {\begin{matrix}{x,\; {{{if}\mspace{14mu} x} \geq 0}} \\{{- x},\; {{{if}\mspace{14mu} x} \leq 0}}\end{matrix}.} $

The second threshold may be 200%-300% of the first threshold. The secondthreshold may be at least 50% of the maximum voltage one incident X-rayphoton may generate in the diode or resistor. For example, the secondthreshold may be 100 mV, 150 mV, 200 mV, 250 mV or 300 mV. The secondvoltage comparator 302 and the first voltage comparator 310 may be thesame component. Namely, the system 121 may have one voltage comparatorthat can compare a voltage with two different thresholds at differenttimes.

The first voltage comparator 301 or the second voltage comparator 302may include one or more op-amps or any other suitable circuitry. Thefirst voltage comparator 301 or the second voltage comparator 302 mayhave a high speed to allow the system 121 to operate under a high fluxof incident X-ray. However, having a high speed is often at the cost ofpower consumption.

The counter 320 is configured to register a number of X-ray photonsreaching the diode or resistor. The counter 320 may be a softwarecomponent (e.g., a number stored in a computer memory) or a hardwarecomponent (e.g., a 4017 IC and a 7490 IC).

The controller 310 may be a hardware component such as a microcontrollerand a microprocessor. The controller 310 is configured to start a timedelay from a time at which the first voltage comparator 301 determinesthat the absolute value of the voltage equals or exceeds the absolutevalue of the first threshold (e.g., the absolute value of the voltageincreases from below the absolute value of the first threshold to avalue equal to or above the absolute value of the first threshold). Theabsolute value is used here because the voltage may be negative orpositive, depending on whether the voltage of the cathode or the anodeof the diode or which electrical contact is used. The controller 310 maybe configured to keep deactivated the second voltage comparator 302, thecounter 320 and any other circuits the operation of the first voltagecomparator 301 does not require, before the time at which the firstvoltage comparator 301 determines that the absolute value of the voltageequals or exceeds the absolute value of the first threshold. The timedelay may expire before or after the voltage becomes stable, i.e., therate of change of the voltage is substantially zero. The phase “the rateof change of the voltage is substantially zero” means that temporalchange of the voltage is less than 0.1%/ns. The phase “the rate ofchange of the voltage is substantially non-zero” means that temporalchange of the voltage is at least 0.1%/ns.

The controller 310 may be configured to activate the second voltagecomparator during (including the beginning and the expiration) the timedelay. In an embodiment, the controller 310 is configured to activatethe second voltage comparator at the beginning of the time delay. Theterm “activate” means causing the component to enter an operationalstate (e.g., by sending a signal such as a voltage pulse or a logiclevel, by providing power, etc.). The term “deactivate” means causingthe component to enter a non-operational state (e.g., by sending asignal such as a voltage pulse or a logic level, by cut off power,etc.). The operational state may have higher power consumption (e.g., 10times higher, 100 times higher, 1000 times higher) than thenon-operational state. The controller 310 itself may be deactivateduntil the output of the first voltage comparator 301 activates thecontroller 310 when the absolute value of the voltage equals or exceedsthe absolute value of the first threshold.

The controller 310 may be configured to cause the number registered bythe counter 320 to increase by one, if, during the time delay, thesecond voltage comparator 302 determines that the absolute value of thevoltage equals or exceeds the absolute value of the second threshold.

The controller 310 may be configured to cause the voltmeter 306 tomeasure the voltage upon expiration of the time delay. The controller310 may be configured to connect the electrode to an electrical ground,so as to reset the voltage and discharge any charge carriers accumulatedon the electrode. In an embodiment, the electrode is connected to anelectrical ground after the expiration of the time delay. In anembodiment, the electrode is connected to an electrical ground for afinite reset time period. The controller 310 may connect the electrodeto the electrical ground by controlling the switch 305. The switch maybe a transistor such as a field-effect transistor (FET).

In an embodiment, the system 121 has no analog filter network (e.g., aRC network). In an embodiment, the system 121 has no analog circuitry.

The voltmeter 306 may feed the voltage it measures to the controller 310as an analog or digital signal.

The system 121 may include a capacitor module 309 electrically connectedto the electrode of the diode 300 or which electrical contact, whereinthe capacitor module is configured to collect charge carriers from theelectrode. The capacitor module can include a capacitor in the feedbackpath of an amplifier. The amplifier configured as such is called acapacitive transimpedance amplifier (CTIA). CTIA has high dynamic rangeby keeping the amplifier from saturating and improves thesignal-to-noise ratio by limiting the bandwidth in the signal path.Charge carriers from the electrode accumulate on the capacitor over aperiod of time (“integration period”) (e.g., as shown in FIG. 8, betweent₀ to t₁, or t₁-t₂). After the integration period has expired, thecapacitor voltage is sampled and then reset by a reset switch. Thecapacitor module can include a capacitor directly connected to theelectrode.

FIG. 8 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by charge carriersgenerated by an X-ray photon incident on the diode or the resistor, anda corresponding temporal change of the voltage of the electrode (lowercurve). The voltage may be an integral of the electric current withrespect to time. At time t₀, the X-ray photon hits the diode or theresistor, charge carriers start being generated in the diode or theresistor, electric current starts to flow through the electrode of thediode or the resistor, and the absolute value of the voltage of theelectrode or electrical contact starts to increase. At time t₁, thefirst voltage comparator 301 determines that the absolute value of thevoltage equals or exceeds the absolute value of the first threshold V1,and the controller 310 starts the time delay TD1 and the controller 310may deactivate the first voltage comparator 301 at the beginning of TD1.If the controller 310 is deactivated before t₁, the controller 310 isactivated at t₁. During TD1, the controller 310 activates the secondvoltage comparator 302. The term “during” a time delay as used heremeans the beginning and the expiration (i.e., the end) and any time inbetween. For example, the controller 310 may activate the second voltagecomparator 302 at the expiration of TD1. If during TD1, the secondvoltage comparator 302 determines that the absolute value of the voltageequals or exceeds the absolute value of the second threshold at time t₂,the controller 310 causes the number registered by the counter 320 toincrease by one. At time t_(e), all charge carriers generated by theX-ray photon drift out of the X-ray absorption layer 110. At time t_(s),the time delay TD1 expires. In the example of FIG. 8, time t_(s) isafter time t_(e); namely TD1 expires after all charge carriers generatedby the X-ray photon drift out of the X-ray absorption layer 110. Therate of change of the voltage is thus substantially zero at t_(s). Thecontroller 310 may be configured to deactivate the second voltagecomparator 302 at expiration of TD1 or at t₂, or any time in between.

The controller 310 may be configured to cause the voltmeter 306 tomeasure the voltage upon expiration of the time delay TD1. In anembodiment, the controller 310 causes the voltmeter 306 to measure thevoltage after the rate of change of the voltage becomes substantiallyzero after the expiration of the time delay TD1. The voltage at thismoment is proportional to the amount of charge carriers generated by anX-ray photon, which relates to the energy of the X-ray photon. Thecontroller 310 may be configured to determine the energy of the X-rayphoton based on voltage the voltmeter 306 measures. One way to determinethe energy is by binning the voltage. The counter 320 may have asub-counter for each bin. When the controller 310 determines that theenergy of the X-ray photon falls in a bin, the controller 310 may causethe number registered in the sub-counter for that bin to increase byone. Therefore, the system 121 may be able to detect an X-ray image andmay be able to resolve X-ray photon energies of each X-ray photon.

After TD1 expires, the controller 310 connects the electrode to anelectric ground for a reset period RST to allow charge carriersaccumulated on the electrode to flow to the ground and reset thevoltage. After RST, the system 121 is ready to detect another incidentX-ray photon. Implicitly, the rate of incident X-ray photons the system121 can handle in the example of FIG. 8 is limited by 1/(TD1+RST). Ifthe first voltage comparator 301 has been deactivated, the controller310 can activate it at any time before RST expires. If the controller310 has been deactivated, it may be activated before RST expires.

FIG. 9 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by noise (e.g., darkcurrent, background radiation, scattered X-rays, fluorescent X-rays,shared charges from adjacent pixels), and a corresponding temporalchange of the voltage of the electrode (lower curve), in the system 121operating in the way shown in FIG. 8. At time t₀, the noise begins. Ifthe noise is not large enough to cause the absolute value of the voltageto exceed the absolute value of V1, the controller 310 does not activatethe second voltage comparator 302. If the noise is large enough to causethe absolute value of the voltage to exceed the absolute value of V1 attime t₁ as determined by the first voltage comparator 301, thecontroller 310 starts the time delay TD1 and the controller 310 maydeactivate the first voltage comparator 301 at the beginning of TD1.During TD1 (e.g., at expiration of TD1), the controller 310 activatesthe second voltage comparator 302. The noise is very unlikely largeenough to cause the absolute value of the voltage to exceed the absolutevalue of V2 during TD1. Therefore, the controller 310 does not cause thenumber registered by the counter 320 to increase. At time t_(e), thenoise ends. At time t_(s), the time delay TD1 expires. The controller310 may be configured to deactivate the second voltage comparator 302 atexpiration of TD1. The controller 310 may be configured not to cause thevoltmeter 306 to measure the voltage if the absolute value of thevoltage does not exceed the absolute value of V2 during TD1. After TD1expires, the controller 310 connects the electrode to an electric groundfor a reset period RST to allow charge carriers accumulated on theelectrode as a result of the noise to flow to the ground and reset thevoltage. Therefore, the system 121 may be very effective in noiserejection.

FIG. 10 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by charge carriersgenerated by an X-ray photon incident on the diode or the resistor, anda corresponding temporal change of the voltage of the electrode (lowercurve), when the system 121 operates to detect incident X-ray photons ata rate higher than 1/(TD1+RST). The voltage may be an integral of theelectric current with respect to time. At time t₀, the X-ray photon hitsthe diode or the resistor, charge carriers start being generated in thediode or the resistor, electric current starts to flow through theelectrode of the diode or the electrical contact of resistor, and theabsolute value of the voltage of the electrode or the electrical contactstarts to increase. At time t₁, the first voltage comparator 301determines that the absolute value of the voltage equals or exceeds theabsolute value of the first threshold V1, and the controller 310 startsa time delay TD2 shorter than TD1, and the controller 310 may deactivatethe first voltage comparator 301 at the beginning of TD2. If thecontroller 310 is deactivated before t₁, the controller 310 is activatedat t₁. During TD2 (e.g., at expiration of TD2), the controller 310activates the second voltage comparator 302. If during TD2, the secondvoltage comparator 302 determines that the absolute value of the voltageequals or exceeds the absolute value of the second threshold at time t₂,the controller 310 causes the number registered by the counter 320 toincrease by one. At time t_(e), all charge carriers generated by theX-ray photon drift out of the X-ray absorption layer 110. At time t_(h),the time delay TD2 expires. In the example of FIG. 10, time t_(h) isbefore time t_(e); namely TD2 expires before all charge carriersgenerated by the X-ray photon drift out of the X-ray absorption layer110. The rate of change of the voltage is thus substantially non-zero att_(h). The controller 310 may be configured to deactivate the secondvoltage comparator 302 at expiration of TD2 or at t₂, or any time inbetween.

The controller 310 may be configured to extrapolate the voltage at t_(e)from the voltage as a function of time during TD2 and use theextrapolated voltage to determine the energy of the X-ray photon.

After TD2 expires, the controller 310 connects the electrode to anelectric ground for a reset period RST to allow charge carriersaccumulated on the electrode to flow to the ground and reset thevoltage. In an embodiment, RST expires before t_(e). The rate of changeof the voltage after RST may be substantially non-zero because allcharge carriers generated by the X-ray photon have not drifted out ofthe X-ray absorption layer 110 upon expiration of RST before t_(e). Therate of change of the voltage becomes substantially zero after t_(e) andthe voltage stabilized to a residue voltage VR after t_(e). In anembodiment, RST expires at or after t_(e), and the rate of change of thevoltage after RST may be substantially zero because all charge carriersgenerated by the X-ray photon drift out of the X-ray absorption layer110 at t_(e). After RST, the system 121 is ready to detect anotherincident X-ray photon. If the first voltage comparator 301 has beendeactivated, the controller 310 can activate it at any time before RSTexpires. If the controller 310 has been deactivated, it may be activatedbefore RST expires.

FIG. 11 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by noise (e.g., darkcurrent, background radiation, scattered X-rays, fluorescent X-rays,shared charges from adjacent pixels), and a corresponding temporalchange of the voltage of the electrode (lower curve), in the system 121operating in the way shown in FIG. 10. At time t₀, the noise begins. Ifthe noise is not large enough to cause the absolute value of the voltageto exceed the absolute value of V1, the controller 310 does not activatethe second voltage comparator 302. If the noise is large enough to causethe absolute value of the voltage to exceed the absolute value of V1 attime t₁ as determined by the first voltage comparator 301, thecontroller 310 starts the time delay TD2 and the controller 310 maydeactivate the first voltage comparator 301 at the beginning of TD2.During TD2 (e.g., at expiration of TD2), the controller 310 activatesthe second voltage comparator 302. The noise is very unlikely largeenough to cause the absolute value of the voltage to exceed the absolutevalue of V2 during TD2. Therefore, the controller 310 does not cause thenumber registered by the counter 320 to increase. At time t_(e), thenoise ends. At time t_(h), the time delay TD2 expires. The controller310 may be configured to deactivate the second voltage comparator 302 atexpiration of TD2. After TD2 expires, the controller 310 connects theelectrode to an electric ground for a reset period RST to allow chargecarriers accumulated on the electrode as a result of the noise to flowto the ground and reset the voltage. Therefore, the system 121 may bevery effective in noise rejection.

FIG. 12 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by charge carriersgenerated by a series of X-ray photons incident on the diode or theresistor, and a corresponding temporal change of the voltage of theelectrode (lower curve), in the system 121 operating in the way shown inFIG. 10 with RST expires before t_(e). The voltage curve caused bycharge carriers generated by each incident X-ray photon is offset by theresidue voltage before that photon. The absolute value of the residuevoltage successively increases with each incident photon. When theabsolute value of the residue voltage exceeds V1 (see the dottedrectangle in FIG. 12), the controller starts the time delay TD2 and thecontroller 310 may deactivate the first voltage comparator 301 at thebeginning of TD2. If no other X-ray photon incidence on the diode or theresistor during TD2, the controller connects the electrode to theelectrical ground during the reset time period RST at the end of TD2,thereby resetting the residue voltage. The residue voltage thus does notcause an increase of the number registered by the counter 320.

FIG. 13 schematically shows a system comprising the semiconductor X-raydetector 100 described herein. The system may be used for medicalimaging such as chest X-ray radiography, abdominal X-ray radiography,etc. The system comprises an X-ray source 1201. X-ray emitted from theX-ray source 1201 penetrates an object 1202 (e.g., a human body partsuch as chest, limb, abdomen), is attenuated by different degrees by theinternal structures of the object 1202 (e.g., bones, muscle, fat andorgans, etc.), and is projected to the semiconductor X-ray detector 100.The semiconductor X-ray detector 100 forms an image by detecting theintensity distribution of the X-ray.

FIG. 14 schematically shows a system comprising the semiconductor X-raydetector 100 described herein. The system may be used for medicalimaging such as dental X-ray radiography. The system comprises an X-raysource 1301. X-ray emitted from the X-ray source 1301 penetrates anobject 1302 that is part of a mammal (e.g., human) mouth. The object1302 may include a maxilla bone, a palate bone, a tooth, the mandible,or the tongue. The X-ray is attenuated by different degrees by thedifferent structures of the object 1302 and is projected to thesemiconductor X-ray detector 100. The semiconductor X-ray detector 100forms an image by detecting the intensity distribution of the X-ray.Teeth absorb X-ray more than dental caries, infections, periodontalligament. The dosage of X-ray radiation received by a dental patient istypically small (around 0.150 mSv for a full mouth series).

FIG. 15 schematically shows a cargo scanning or non-intrusive inspection(NII) system comprising the semiconductor X-ray detector 100 describedherein. The system may be used for inspecting and identifying goods intransportation systems such as shipping containers, vehicles, ships,luggage, etc. The system comprises an X-ray source 1401. X-ray emittedfrom the X-ray source 1401 may backscatter from an object 1402 (e.g.,shipping containers, vehicles, ships, etc.) and be projected to thesemiconductor X-ray detector 100. Different internal structures of theobject 1402 may backscatter X-ray differently. The semiconductor X-raydetector 100 forms an image by detecting the intensity distribution ofthe backscattered X-ray and/or energies of the backscattered X-rayphotons.

FIG. 16 schematically shows another cargo scanning or non-intrusiveinspection (NII) system comprising the semiconductor X-ray detector 100described herein. The system may be used for luggage screening at publictransportation stations and airports. The system comprises an X-raysource 1501. X-ray emitted from the X-ray source 1501 may penetrate apiece of luggage 1502, be differently attenuated by the contents of theluggage, and projected to the semiconductor X-ray detector 100. Thesemiconductor X-ray detector 100 forms an image by detecting theintensity distribution of the transmitted X-ray. The system may revealcontents of luggage and identify items forbidden on publictransportation, such as firearms, narcotics, edged weapons, flammables.

FIG. 17 schematically shows a full-body scanner system comprising thesemiconductor X-ray detector 100 described herein. The full-body scannersystem may detect objects on a person's body for security screeningpurposes, without physically removing clothes or making physicalcontact. The full-body scanner system may be able to detect non-metalobjects. The full-body scanner system comprises an X-ray source 1601.X-ray emitted from the X-ray source 1601 may backscatter from a human1602 being screened and objects thereon, and be projected to thesemiconductor X-ray detector 100. The objects and the human body maybackscatter X-ray differently. The semiconductor X-ray detector 100forms an image by detecting the intensity distribution of thebackscattered X-ray. The semiconductor X-ray detector 100 and the X-raysource 1601 may be configured to scan the human in a linear orrotational direction.

FIG. 18 schematically shows an X-ray computed tomography (X-ray CT)system. The X-ray CT system uses computer-processed X-rays to producetomographic images (virtual “slices”) of specific areas of a scannedobject. The tomographic images may be used for diagnostic andtherapeutic purposes in various medical disciplines, or for flawdetection, failure analysis, metrology, assembly analysis and reverseengineering. The X-ray CT system comprises the semiconductor X-raydetector 100 described herein and an X-ray source 1701. Thesemiconductor X-ray detector 100 and the X-ray source 1701 may beconfigured to rotate synchronously along one or more circular or spiralpaths.

FIG. 19 schematically shows an electron microscope. The electronmicroscope comprises an electron source 1801 (also called an electrongun) that is configured to emit electrons. The electron source 1801 mayhave various emission mechanisms such as thermionic, photocathode, coldemission, or plasmas source. The emitted electrons pass through anelectronic optical system 1803, which may be configured to shape,accelerate, or focus the electrons. The electrons then reach a sample1802 and an image detector may form an image therefrom. The electronmicroscope may comprise the semiconductor X-ray detector 100 describedherein, for performing energy-dispersive X-ray spectroscopy (EDS). EDSis an analytical technique used for the elemental analysis or chemicalcharacterization of a sample. When the electrons incident on a sample,they cause emission of characteristic X-rays from the sample. Theincident electrons may excite an electron in an inner shell of an atomin the sample, ejecting it from the shell while creating an electronhole where the electron was. An electron from an outer, higher-energyshell then fills the hole, and the difference in energy between thehigher-energy shell and the lower energy shell may be released in theform of an X-ray. The number and energy of the X-rays emitted from thesample can be measured by the semiconductor X-ray detector 100.

The semiconductor X-ray detector 100 described here may have otherapplications such as in an X-ray telescope, X-ray mammography,industrial X-ray defect detection, X-ray microscopy or microradiography,X-ray casting inspection, X-ray non-destructive testing, X-ray weldinspection, X-ray digital subtraction angiography, etc. It may besuitable to use this semiconductor X-ray detector 100 in place of aphotographic plate, a photographic film, a PSP plate, an X-ray imageintensifier, a scintillator, or another semiconductor X-ray detector.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

1. A method of making an apparatus suitable for detecting x-ray, themethod comprising: obtaining a substrate having a first surface and asecond surface, wherein the substrate comprises an electronics system inor on the substrate, wherein the substrate comprises a plurality ofelectric contacts are on the first surface; obtaining a first chipcomprising a first X-ray absorption layer, wherein the first X-rayabsorption layer comprises an electrode; bonding the first chip to thesubstrate such that the electrode of the first X-ray absorption layer iselectrically connected to at least one of the electrical contacts; andobtaining a second chip comprising a second X-ray absorption layer,wherein the second X-ray absorption layer comprises an electrode, andbonding the second chip to the substrate such that the electrode of thesecond X-ray absorption layer is electrically connected to at least oneof the electrical contacts.
 2. The method of claim 1, further comprisingmounting a backing substrate to the first chip such that the first chipis sandwiched between the backing substrate and the substrate. 3.(canceled)
 4. The method of claim 1, wherein a gap between the firstchip and the second chip is less than 100 microns.
 5. The method ofclaim 1, wherein the first chip is smaller in area than the substrate.6. The method of claim 1, wherein a ratio between a thermal expansioncoefficient of the first chip and a thermal expansion coefficient of thesubstrate is two or more.
 7. The method of claim 1, wherein the X-rayabsorption layer comprises silicon, germanium, GaAs, CdTe, CdZnTe, or acombination thereof.
 8. The method of claim 1, wherein the X-rayabsorption layer is doped with chromium.
 9. The method of claim 1,wherein the X-ray absorption layer has a thickness of 200 microns orless.
 10. The method of claim 1, wherein the first chip comprises aredistribution layer (RDL) on the second surface.
 11. The method ofclaim 1, wherein the first chip comprises a via, wherein the via extendsfrom the first surface to the second surface.
 12. The method of claim 1,wherein the electronics system comprises: a first voltage comparatorconfigured to compare a voltage of the electrode to a first threshold; asecond voltage comparator configured to compare the voltage to a secondthreshold; a counter configured to register a number of X-ray photonsreaching the X-ray absorption layer; a controller; wherein thecontroller is configured to start a time delay from a time at which thefirst voltage comparator determines that an absolute value of thevoltage equals or exceeds an absolute value of the first threshold;wherein the controller is configured to activate the second voltagecomparator during the time delay; wherein the controller is configuredto cause the number registered by the counter to increase by one, if thesecond voltage comparator determines that an absolute value of thevoltage equals or exceeds an absolute value of the second threshold. 13.The method of claim 12, wherein the electronics system further comprisesa capacitor module electrically connected to the electrode of the firstX-ray absorption layer, wherein the capacitor module is configured tocollect charge carriers from the electrode of the first X-ray absorptionlayer.
 14. The method of claim 12, wherein the controller is configuredto activate the second voltage comparator at a beginning or expirationof the time delay.
 15. The method of claim 2, wherein the electronicssystem further comprises a voltmeter, wherein the controller isconfigured to cause the voltmeter to measure the voltage upon expirationof the time delay.
 16. The method of claim 15, wherein the controller isconfigured to determine an X-ray photon energy based on a value of thevoltage measured upon expiration of the time delay.
 17. The method ofclaim 12, wherein the controller is configured to connect the electrodeof the first X-ray absorption layer to an electrical ground.
 18. Themethod of claim 12, wherein a rate of change of the voltage issubstantially zero at expiration of the time delay.
 19. The method ofclaim 12, wherein a rate of change of the voltage is substantiallynon-zero at expiration of the time delay.
 20. The method of claim 1,wherein the X-ray absorption layer comprises a diode.